Cell Membranes
Osmosis and Diffusion
Functions of Membranes
1. Protect cell
2. Control incoming and outgoing substances
3. Maintain ion concentrations of various
substances
4. Selectively permeable - allows some
molecules in, others are kept out
Phospholipid Bilayer
Fluid Mosaic Model
Blood-Brain Barrier
• Allows some substances into the
brain, but screens out toxins and
bacteria
• Substances allowed to cross include:
water, CO2, Glucose, O2, Amino Acids,
Alcohol, and antihistamines. HIV and
bacterial meningitis can cross the
barrier.
Solutions
• Solutions are made of solute and a solvent
• Solvent - the liquid into which the solute is
poured and dissolved. We will use water
as our solvent today.
• Solute - substance that is dissolved or put
into the solvent. Salt and sucrose are
solutes.
Methods of Transport Across
Membranes
1. Diffusion -passive transport – no energy
expended
2. Osmosis - Passive transport of water
across membrane
3. Facilitated Diffusion - Use of proteins to carry polar
molecules or ions across
4. Active Transport- requires energy to transport
molecules against a concentration
gradient – energy is in the form of
ATP
Diffusion
• Movement of molecules from an area of
high concentration to an area of low
concentration.
• Movement from one side of a membrane
to another, un-facilitated
Passive Processes
Diffusion vs. Osmosis
• 1. Simple Diffusion
– Gases; nonpolar; lipid soluble
substances (fat-soluble vitamins)
• 2. Facilitated Diffusion
– Sugars (glucose; amino acids;
ions)
– Integral or Transmembrane
proteins
• Channel or carrier proteins
• 3. Osmosis
– Movement of water dependent
on solute concentration
(extracellular/intracellular)
Diffusion
Osmosis
Tonicity is a relative term
• Hypotonic Solution - One solution has a
lower concentration of solute than
another.
• Hypertonic Solution - one solution has a
higher concentration of solute than
another.
• Isotonic Solution - both solutions have
same concentrations of solute.
Plant and Animal Cells put into
various solutions
Tonicity
• Measure of the ability of a solution to cause a
change in cell shape or tone caused by osmotic
flow of water
• Why does osmosis occur?
– Water concentration differences
– Solute concentration affects water concentration
• Dependent on the number (concentration)
– Osmolarity
– Permeability of solute molecules
• Permeable to all solute molecules – equilibrium
• If membrane is impermeable (see U-tube)
Water Movement and Solute Concentration
Reminder
• Concentration- The
amount of solute per
unit of solvent.
• Osmosis is the flow
of water down the
concentration
gradient (from an area of
high concentration to an
area of low concentration)
Water never stops moving, even when equilibrium
has been reached.
Isotonic
• The net movement of
water across the
semipermeable
membrane is equal.
• The amount of water
entering a cell is the
same as the amount
leaving the cell
• Isotonic Plant cells are
Flaccid.
Water never stops moving, even when equilibrium
has been reached.
Hypotonic
• There is more substances than
water inside the cell. The
concentration will try to even
out.
• Water will enter the cell. This
causes the cell to swell (get
larger) until it may burst.
• Hypotonic Plant cells are
Turgid.
• The Process of swelling animal
cells is called hemolysis
PUTTING A CELL IN PLAIN
WATER
Hypertonic
• There is less substances than
water inside the cell. The
concentration will try to even
out.
• Water will leave the cell. This
causes the cell to shrink (get
smaller)
• Hypertonic Plant cells are
Plasmolyzed.
• The Process of shrinking
animals cells is called
crenation
PUTTING A CELL IN SALT
WATER
Tonicity
Concentration
Water
of
(outside of
Solute (outside
cell)
of cell)
Net
End
Movement Product
Isotonic
Same as
cell
Same
as cell
None
Hypotonic
Less than
cell
More
Cell gains
than cell water
Swells,
turgor
pressure
Hypertonic More than
Less
Cell loses
than cell water
Shrinks
plasmoly
sis
Tonicity
of
Solution
cell
None
Practice
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter21/animation__hemolysis_and_crenatio
n.html
• Lets watch a clip and answer questions about
red blood cells in different solutions.
Types of Transport